This invention relates generally to the fabrication of semiconductor devices using pattern lithography and, more specifically, to the control of pattern edge profiles in photoresist layers. The art of photolithography in the fine patterning of semiconductor substrates is well known. Examples of applications of photolithography are the formation of integrated circuits and the formation of magnetic recording heads.
In a conventional pattern forming process, a substrate is coated with a layer of a radiation sensitive composition called resist. Photoresist is the term used for resist materials that are sensitive to optical radiation. Similarly, electron resist materials are sensitive to electron radiation. The resist layer is exposed to electromagnetic radiation (i.e. light, X-rays, gamma rays, ions or electrons) to change the solubility characteristics of selected portions of the resist layer. A relief image is formed by action of a solvent to remove the more soluble portions of the radiation sensitive resist layer. Pattern formation is a result of the differential solubility between irradiated and non-irradiated regions. These solubility changes are produced by either bondbreaking (chain scission) or bond formation (chain crosslinking) in a polymeric resist. A positive acting resist will become more soluble via chain scission when irradiated. A negative resist will crosslink and become more insoluble when irradiated. Thus a positive resist is one in which the material remaining after exposure and development corresponds to the opaque portions of a mask used for selective radiation of the resist. A negative resist is one in which the material remaining after exposure and development corresponds to the transparent portions of the mask.
A critical operation in high resolution lithography is the transfer of the pattern defined in the resist, to a layer of other material in or on the substrate. There are many different kinds of pattern transfer processes, which may be additive or subtractive in nature. Subtractive processes involve an etching away of material using dry plasma, a chemical solution, or an ion beam. For example, FIG. 1a shows a substrate 20 with a layer 21 to be patterned, coated with resist 22. FIG. 1b shows the resist layer 22 being selectively exposed to radiation, indicated by the arrows 23, which degrades the resist in the area exposed, such as the area 24. The resist is next immersed in a weak solvent, which dissolves away the exposed region 24, leaving the relief pattern shown in FIG. 1c. The resist layer 22 than acts as a protective mask for the subsequent etching away of the material in layer 21 to be patterned, as shown in FIG. 1d. Remaining portions of the resist layer 22 are then stripped away in a strong solvent, leaving the desired pattern in layer 21 as shown in FIG. 1e.
Additive processes are those where material is deposited (via evaporation, electroplating, or ion implantation) after the resist has been patterned. In an additive or so called lift-off process, a metal is deposited (added) after resist patterning and then the resist is stripped off, leaving metal in the windowed areas of the resist. Such a process is shown in FIGS. 2a through 2e. In FIG. 2a, the substrate 20 is bare except for the resist layer 22. The resist is then exposed to patterned radiation 23, as shown in FIG. 2b. The exposed resist is then developed, i.e. dissolved in the exposed patterned areas, such as the area 24, as shown in FIG. 2c. A metal or other material 25 to be patterned is deposited on top of the resist layer 22. The metal adheres to the substrate 20 in the patterned regions, such as 24, as shown in FIG. 2d. The resist layer 22 is then removed (dissolved), carrying away the metal layer 25 everywhere except in the patterned regions 24, as shown in FIG. 2e. For this lift-off additive process, an undercut resist profile is desired to provide a clean discontinuity of the deposited metal layer 25.
Semiconductor fabrication processes have utilized many techniques to control the edge profile of resists. In some additive processes it is desirable to have an undercut edge profile. In some etching processes it is desirable to have a vertical wall profile. And in some processes a tapered edge slope is required. Prior to the present invention, changing the resist wall slope has required additional processing steps and multiple resist layers, adding to the complexity and cost of fabrication. Furthermore these additional processing steps, for example to achieve an undercut profile or vertical side walls, often decrease the overall manufacturing yield of the devices, i.e. the percentage of fully operational devices produced.
Electron beam pattern exposure typically provides an undercut profile when utilizing positive resists. Photon or optical exposure typically results in a tapered edge profile. Although an undercut profile is preferred for a lift-off process, it is not preferred for an electroplating process where an upward force can be generated that will lift up narrow lines of the metalized layer. It is therefore highly desirable to have some means for controlling the edge slope and resist profiles in high resolution lithography. Numerous workers in the field have devised processing means to control edge profiles. Without exception, these prior art techniques add complexity to the fabrication process, such as additional coating steps, multiple resist layers and so forth. In general, this additional complexity tends to degrade overall process yields.
An example of the prior art, shown in FIG. 3a, is a typical patterning step (not shown to scale). A substrate 20 is coated with resist 22 and exposed in the crosshatched area 24 to an exposure level designated Dose A. In an ideal process the resist profile (after development) will look as depicted in FIG. 3b, where the resist is removed in the area exposed 26, with the unexposed resist exhibiting vertical side walls 29 down to the substrate 20. In practice, however, when the resist 22 is being developed the developer acts as a weak solvent, also attacking the unexposed portions 22 of the resist, as well as the exposed portion 24. FIG. 3c shows how, during development, the corner edges of the resist 22 are attacked by the developer 28, both along the inside walls and on the top edge of the resist. This results in sloped or tapered sidewalls 30 in the developed resist image, as shown in FIG. 3d.
Undercut profiles may be obtained utilizing an electron beam to provide the primary patterning exposure, because the energy absorption in the resist layer during exposure is not linear, but reaches a maximum at about one-third of the beam penetration range. FIG. 4 shows a typical dose distribution curve as a function of depth into the resist. In optical exposure of photoresists, however, energy absorption is highest at the top of the resist layer and lowest at the interface between the resist and the underlying substrate, due to light attenuation in the resist. It is therefore ordinarily impossible to obtain an undercut profile or even a vertical profile with normal optical exposure and normal development of the commonly used positive photoresists.
One prior art technique utilizing a blanket exposure step requires two resist layers, including an upper layer formed on an underlying resist layer. The upper resist layer is first patterned and developed, and then acts as a mask for a flood ultraviolet exposure of the underlying resist layer. By adjusting the solubilities of the top and bottom layers, an undercut or tapered edge slope may be attained. The problem with this technique is that it is difficult to coat one resist on top of the other without distorting the underlaying resist. In some cases an additional (metal or inorganic) layer is added between the two resist layers to act as a barrier interface to separate the two different chemistries. This adds extra processing steps, with consequential yield loss.
It is the objective of this invention to provide an easily applied and controllable method for achieving defined edge profiles in resist. It is further the objective of this invention to provide a means of controlling edge profiles independent of the primary patterning step, without requiring any additional resist layers or additional processing steps other than a blanket exposure by a uniform electron beam. This blanket exposure can be performed before or after the pattern exposure step but prior to the development of the resist image. The object of this invention is to provide a means of controlling the edge profile of any resist pattern (exposed optically or by other means) by a simple blanket exposure with an electron beam.